Seroepidemiology of peste des petits ruminants virus in small ruminants in selected districts in Northwest Ethiopia

Abstract Background Peste des petits ruminants (PPR) is one of the most severe diseases of small ruminants, causing the loss of millions of dollars annually. A cross‐sectional study was conducted to determine the seroepidemiology of peste des petits ruminants virus (PPRV) in unvaccinated sheep and goats in selected districts in Northwest Ethiopia. Objectives The study was designed to investigate the epidemiology of PPRV in unvaccinated sheep and goats and risk factors in the study areas. Methods A multi‐stage sampling was used to select study districts, villages and households with a random sampling approach. Study animals (403 sheep and goats) older than 5 months were selected with a systematic random sampling approach. From the animals, blood samples were aseptically collected and PPRV antibodies from the serum were analysed with enzyme‐linked immunosorbent assay (ELISA). Results The overall seroprevalence of antibodies to PPRV was 32.5% in both species. It was higher in goats with a prevalence of 34.7% than in sheep (28.3%). District, herd size, sex, animal origin and grazing management were significantly associated with seropositivity of animals to PPRV antibodies. If an animal was from the Dangur district, it had 2.6 times higher chance of being positive than in the Dibati district (OR = 2.6, p = 0.01 and 95% CI = 1.2– 5.6). Herd size was also significantly correlated with the seropositivity with (OR = 4, p = 0.001, and 95% CI = 1.8–9). Also, male animals had 1.7 times higher chance of being positive than females. Further, if an animal comes from the market, it has 2.7 times higher chance of being positive compared to animals born and raised on the farm. Conclusions The seropositivity indicates that the disease is circulating in the study areas. Hence, preventive approaches, like vaccination campaigns and strict biosecurity measures, are highly advised to avoid the catastrophic impact of the diseases if an outbreak occurs.


INTRODUCTION
Sheep and goats are valuable livestock for food and nutrition security for smallholder farmers because of their high reproductive capacity, excellent adaptation to arid environments and low budget price to start a farm. Following population growth, urbanisation and improved living standards, the demand for animal-derived food is increasing, and as a result, there is a need for sustainable intensification in the small ruminant sector (Markos Tibbo, 2006). Unfortunately, various challenges, such as infectious diseases, affect the production of sheep and goats worldwide and in Africa. Among the diseases affecting sheep and goat production, peste des petits ruminants (PPR) is one of the most severe diseases, with high morbidity and mortality rates (Baron et al., 2016).
It is a viral disease characterised by profuse nasal discharge, coughing, stomatitis, fever and diarrhoea in infected animals (Pradère, 2014).
It is a transboundary animal disease causing a substantial economic burden, especially in Africa (Diallo, 2006).
The virus was first reported in the Ivory Coast, West Africa, in 1942 and has spread to other areas (FAO, 2009). It is classified under the family Paramyxoviridae and the genus morbillivirus, closely related to the rinderpest virus (Sen et al., 2010). Recently it was renamed Small Ruminant Morbillivirus (SRMV). In Ethiopia, clinical signs indicative of possible PPRV infection in goatherds in the Afar were first noted in 1977 in goatherds of the Afar regional state but confirmed for the first time in Ethiopia with cDNA probes to have been peste des petits ruminants in 1991 in holding grounds found in Addis Ababa (Roeder et al., 1994). Since then, it has affected the country's pastoral and highland sheep and goat farmers, causing mortalities and poor production potentials of sheep and goats. Reports indicate that it has a seroprevalence of 38% in Afar, 4.6% in Amhara, 8% in Benishangul Gumuz and 1.7% in the Oromia regional state (Abraham et al., 2005;Delil et al., 2012;Megersa et al., 2011;Waret-Szkuta et al., 2008 (Abraham et al., 2005;Delil et al., 2012;Megersa et al., 2011;Waret-Szkuta et al., 2008

Study area
The study was carried out in districts found in Debati, Dangur, Man-

Study population
The study populations were sheep and goats of Gumuz goat breed kept under the traditional farming system with no vaccination history against PPR. Moreover, small ruminants younger than 5 months were excluded from sampling to avoid seropositivity due to maternal antibodies as a confounder. The age of the animals was categorised as young (less than 3 years old) and adult (greater than 3 years old).

Study design and sample size determination
The study design was cross-sectional, and data collection was conducted from November 2019 to July 2020. The sample size was calculated using the formula described by Thrusfield and Christley (2018). Statistical considerations during sample size determination include a 95% confidence interval and 5% desired absolute precision.
Hence, 384 small ruminants (sheep and goats) were required, but we increased the sample size to 403 to allow for mistakes during the TA B L E 1 Descriptive characteristics of study area and animals in Northwest Ethiopia

Sampling strategy
A multi-stage sampling with four hierarchical units was used as a sampling strategy (Dohoo et al., 2003). Districts bordering Amhara regional state were purposively selected because of high contact frequencies with sheep and goats in trade and/or small ruminants kept by nomadic farmers in these areas. Kebeles (the smallest administrative area in Ethiopia), and households, were sampled in random sampling, while animals were selected with a systematic random sampling approach.
In each district, four to six rural kebeles were randomly selected proportionally to the number of kebeles in a district by simple random sampling (lottery method) with 19 rural kebeles. Study animals of both sexes and above 5 months' age groups of sheep and goats were sampled with systematic random sampling. Besides, a mini semi-structured questionnaire through interviews was administered by asking small ruminant animal owners willing to participate in the study and collect samples from their animals to collect information on the occurrence of PPR in their her and the typical clinical signs they observed.

Serum sample collection and analysis
In order to get serum samples for serological detection, 10 ml of blood was collected from each study animal. The collected blood was allowed to clot by placing the samples at room temperature for two consecutive hours without shaking the samples. Then samples were stored horizon-tally for 3-4 h and serum was separated from the vacutainer tube to cryovials and stored at -20 • C until analysed. Samples were analysed using a competitive enzyme-linked immunosorbent assay (c-ELISA) test kit (IDScreen ® PPR Competition, Montpellier, France) following the manufacturer's instructions.

Data management and analysis
The data obtained was cleared, filtered and coded in MS Excel and analysed using STATA version 14 for statistical analysis (STATA, 2014).
Logistic regression was used to quantify the association between the predictor variables and seroprevalence of PPRV antibodies with variables district, species, sex, age, herd size, origin and grazing management. Univariable logistic regression analysis was conducted to reduce the non-important variables with p = 0.25, further model development with stepwise selection in multiple logistic regressions to adjust for confounding and determine their independent effect on PPR seropositivity. A p value less than 0.05 was used to indicate a significant level.

Descriptive results
In

Grazing management
Private land Reference Communal land 2.5 0.001*** 1.6-4 *significant at alpha value less than or equal to 0.05. **significant at alpha value less than or equal to 0.01. ***significant at alpha value less than or equal to 0.001.

Association between seropositivity and risk factors
When modelled with univariable logistic regression, study districts, herd size, origin of animals, and grazing management type were significantly associated with seropositivity (Table 2). After removing variables that have p > 0.25 from the univariable logistic regression analysis, the final model for multivariable logistic regression analysis containing variables like sex, animal origin, herd size, animal origin, and grazing management type was fitted.
Districts, herd size, sex, animal origin and grazing management were significantly associated with seropositivity. An animal from the Dangur district had 2.6 times more chance of being seropositive than the reference group Dibati district (OR = 2.6, p = 0.01 and 95% CI 1.2-5.6). If an animal comes from the Mandura district, it had a chance of 1.6 times higher than Dibati, which was not statistically significant (OR = 1.6, p = 0.25 and 95% CI = 1.2-5.6). Further, animals from Pawe had a 4.3 times higher chance of being seropositive than the reference (OR = 4.3, p = 0.001 and 95% CI = 2-9). Furthermore, herd size was also significantly correlated with the seropositivity of animals to PPRV antibodies. If animals were from large herd sizes, they had an odd of 4 times higher chance of being positive than the reference small herd size (OR = 4, p = 0.001, and 95% CI = 1.8-9). Whereas medium herd size had a numerically higher chance of being positive than small herd size, the difference was not statistically significant.
Sex of animals was also associated with seropositivity to PPRV antibodies, in which male animals had a higher chance of being positive than females. If an animal is male, it has a 70% higher chance of being positive than females, in which the difference is statistically significant (Or = 1.7, p = 0.029 and 95% CI = 1.1-2.8). The origin of animals (bought from the market or born and raised in the household) was the other risk factor with a significant association with seropositivity to PPRV antibodies. If an animal comes from the market, it has a 2.7 times higher chance of being positive compared to animals born and raised on the farm (OR = 2.7, p = 0.001, and 95% CI = 1.6-4.4). Grazing management was also significantly associated with seropositivity. If an animal was allowed to graze in communal grasslands, it had a 2.3 times higher chance of being positive (OR = 2.3, p = 0.01 and 95% CI = 1.2-4.2) ( *significant at alpha value less than or equal to 0.05. **significant at alpha value less than or equal to 0.01. ***significant at alpha value less than or equal to 0.001.
herd-level immunity detection and epidemiological surveys help understand the disease status and design effective disease control programs.
In this study, we reported an overall prevalence of 32.5% PPRV antibodies in both sheep and goats. The seroprevalence detected was higher than previous reports (Gebre et al., 2018) in Southwest Ethiopia reported a seroprevalence of 2.1% (Fentie et al., 2018(Fentie et al., , 2017 in the Amhara Region with an overall prevalence of 18.3% and (Megersa et al., 2011) in the pastoral and agropastoral system in Ethiopia with a prevalence of 7.3% in sheep but higher prevalence of 42% in goats.
In contrast to our findings, (Gizaw et al., 2018) and (Yalew et al., 2019) have reported a higher prevalence of PPR in the Afar region and the Asossa zone of Ethiopia, with a prevalence of 41.5% and 75.5%, respectively. The prevalence of PPR observed in the different settings in Ethiopia could be attributed to the differences in management practices, vaccination status and agroecological differences because agroecology, climate and vegetation determine the spread of the virus during an outbreak (Assefa et al., 2021).
Risk factors like herd size, sex, origin and grazing management were significantly associated with seropositivity. Significantly higher seroprevalence was reported in large herd sizes than in medium and small herd sizes. The higher prevalence in larger herds can be due to health management in larger flocks and high disease transmission within the flock, consequently increasing the number of infected animals. Sexwise, the prevalence was higher in males than females, which was statistically significant. A higher prevalence in males can be due to higher demands on male animals for meat purposes, driving them to the market and exposing them to a higher infection rate than in females, which are relatively kept at home for breeding purposes. Contrary to our study, a study conducted in Afar regional State indicated that all determinant factors were not associated with seroprevalence of PPR (Gizaw et al., 2018). Also, in research conducted in Nigeria, there were no significant differences in the PPRV seroprevalence between male and female animals (Woma et al., 2016).
Furthermore, the origin of animals was also significantly associated with the origin of animals. The odds of seropositivity were 2.72 times higher in animals bought from the market than those born in the household. Animals from the market have a higher risk of exposure to infection due to the intermingling of animals. Besides, Grazing management also contributes significantly to the seroprevalence of animals. If an animal is managed in communal grazing, it has a 2.22 higher chance of being positive than those contained in private grazing. Grazing management in communal grassland is similar to a market in which animals come in contact to increase the risk of exposure. Similarly, in a study conducted in Southern Ethiopia, Flock size and the recent introduction of new animals to the flock were significantly associated with Sero prevalence (Hailegebreal, 2018).

CONCLUSIONS
The

ACKNOWLEDGEMENT
We are grateful to the farmers who participated in this study. Also, we thank developmental agents and veterinarians involved in study unit mobilization and sample collection during the study.

CONFLICT OF INTEREST
We declare that there is no conflict of interest.

FUNDING STATEMENT
This research doesn't receive financial support from public or private entities.

DATA AVAILABILITY STATEMENT
Available upon request of the authors.

ETHICS STATEMENT
The authors confirm that the ethical policies of the journal, as noted on the journal's author guidelines page, have been adhered to and the Ethiopia National Research Ethics Review Guideline for the Care and Use of Animals was followed.